PFM is a scanning probe method, wherein the probe is represented by a particle or a plurality of particles captured in an optical trap. In this case, the optical trap fulfils the role of the cantilever as it is known from the area of scanning force microscopy. Here, the position of the optical trap may be modified by movable or modifiable optical elements in the optical path (acousto-optical deflectors (AOD), galvanically suspended mirrors, spatial light modulators (SLM), piezo-electrically suspended mirrors). Moreover, the position of a captured particle or of a plurality of captured particles may be determined by means of a detection system. In this respect, in principle two methods are distinguished. In the area of Back Focal Plane Interferometry, the interference of the light scattered by the particle with the light transmitted is utilised, in order to obtain the exact three-dimensional position of the particle within the focal volume of the optical trap. The measurement signals are here the intensity of the light transmitted as well as the differential signals of a quadrant photo diode (QPD). Apart from that, there is the method or video microscopy, wherein the detection of the movement of the particles is achieved by means of computer-based image recognition on a video which was recorded for example by means of a CCD camera.
From the U.S. Pat. No. 6,833,923, a photonic force microscope is known wherein the detection system is based on the interferometric measuring method described.
What the detection systems illustrated have in common is that the detection is based on interactions of the light forming the optical trap with the particle within the optical trap. Any interaction of this light with the sample would be undesired, can, however, not be avoided. Thus, any local changes in the index of refraction and/or the absorption coefficient of the sample will lead to changes in the measurement signal used for the localization. This may cause errors in the determination of the position and/or the movement of the particles to occur. The interferences may also be subject to time variations, for example if they are living cells or if the sample is modified by an interaction with the probe.
It is the object of the invention to provide a method and an apparatus for enhancing the possibilities of the scanning probe method for characterising a sample. In particular, any interferences of the sample or their effect on the measurement result, as may be caused by the interaction between the measurement setup and the sample, are supposed to be reduced.